1. Field of the Invention
The present invention relates to a method for producing a planographic printing plate by way of heat mode recording such as by laser light or the like.
2. Description of the Related Art
Recently, in the production of planographic offset printing plates which are widely used in the printing field, attention has been focused on a technique in which a plate production step which is carried out before a printing step is digitized, such that a printing plate is directly formed by laser writing from digital data without the writing being carried out through a lithographic film. For printing of a small number of copies, a waterless planographic printing plate having an ink-repellent layer formed of silicone rubber is suitable since it does not require skill in the printing work. The following various methods have been known for producing a planographic printing plate having an ink-repellent layer by direct laser writing.
For example, EP-0573091B discloses a method in which a plate material having a silicone rubber surface layer is exposed by a YAG laser, and the exposed portions are rubbed off under a dry condition without solvent or are rubbed off while applying a solvent which does not swell silicone rubber, so as to form a waterless plate.
However, when the physical removal of the silicone rubber at the laser-exposed portions by rubbing is conducted under a dry condition, the silicone rubber removed from the exposed portions (i.e., silicone rubber refuse) tends to adhere to the silicone rubber layer at the non-image portions or to the image portions from which the silicone rubber layer has been removed. Further, the silicone rubber layer at the non-image portions tends to be scratched. Therefore, when printing is conducted by using a waterless planographic plate formed by this method, there are the drawbacks that the non-image portions are scratched and stained, and that lack of images is formed at the image portions.
Regarding adhesion of the silicone rubber refuse to the surface of a plate, it is known that there is less adhesion when the physical removal of the silicone rubber at the laser-exposed portions is conducted in the presence of a liquid. For example, Japanese Patent Application Laid-Open (JP-A) No. 50-158405 discloses a method in which a printing original plate having a surface layer of silicone rubber is exposed by a YAG laser which is an infrared laser, and the laser-exposed portions are removed by a solvent (naphtha) treatment (and rubbing) so as to form a planographic plate.
However, when a liquid which swells silicone rubber (for example, a liquid containing as a main component a hydrocarbon-based solvent such as naphtha) is used as the liquid for removal, a drawback arises in that the silicone rubber layer at the non-image portions tends to be scratched even more since the film strength of the silicone rubber decreases.
Therefore, it is suitable to use a liquid which does not swell the silicone rubber layer for the rubbing treatment. Examples of liquids which do not swell the silicone rubber layer are polar solvents such as water, alcohols and the like. However, when a polar solvent is used alone, adhesion of the silicone rubber refuse to the surface of the plate is not sufficiently prevented since the ability of a polar solvent to wet the silicone rubber layer is poor.
In order to improve the ability of the polar solvent to wet the silicone rubber layer, it is effective to add a surfactant to the polar solvent.
On the other hand, when the silicone rubber layer at the laser-exposed portions is removed physically to produce a planographic printing plate on which an image is reproduced uniformly, it is conventional to use an automatic treating machine equipped with a rubbing member.
However, when a liquid containing a surfactant is used in the automatic treating machine, problems arise due to bubbling, such as the generation of bubbles between the rubbing member and the surface of the plate, the generation of bubbles in the treating solution tank, an increase in the load of a liquid feeding pump, which supplies the liquid to the rubbing member and the surface of the plate, due to bubbles entering into the pump, and the like.